Acoustic wave devices are widely used in modern electronics. At a high level, acoustic wave devices include a piezoelectric material in contact with one or more electrodes. Piezoelectric materials acquire a charge when compressed, twisted, or distorted, and similarly compress, twist, or distort when a charge is applied to them. Accordingly, when an alternating electrical signal is applied to the one or more electrodes in contact with the piezoelectric material, a corresponding mechanical signal (i.e., an oscillation or vibration) is transduced therein. Based on the characteristics of the one or more electrodes on the piezoelectric material, the properties of the piezoelectric material, and other factors such as the shape of the acoustic wave device and other structures provided on the device, the mechanical signal transduced in the piezoelectric material exhibits a frequency dependence on the alternating electrical signal. Acoustic wave devices leverage this frequency dependence to provide one or more functions.
Exemplary acoustic wave devices include surface acoustic wave (SAW) resonators and bulk acoustic wave (BAW) resonators, which are increasingly used to form filters used in the transmission and reception of RF signals for communication. For purposes of illustration, FIG. 1 shows details of a conventional SAW resonator 10. The conventional SAW resonator 10 includes a piezoelectric layer 12, an interdigital transducer 14 on a surface of the piezoelectric layer 12, a first reflector structure 16A on the surface of the piezoelectric layer 12 adjacent to the interdigital transducer 14, and a second reflector structure 16B on the surface of the piezoelectric layer 12 adjacent to the interdigital transducer 14 opposite the first reflector structure 16A.
The interdigital transducer 14 includes a first interdigital electrode 18A and a second interdigital electrode 18B, each of which include a number of fingers 20 that are interleaved with one another as shown. A distance between adjacent fingers 20 of the first interdigital electrode 18A and the second interdigital electrode 18B defines an electrode period P of the interdigital transducer 14. A ratio between the cross-sectional area along the surface of the piezoelectric layer 12 occupied by the adjacent fingers 20 and the empty space between the adjacent fingers 20 defines a metallization ratio M of the interdigital transducer 14. The electrode period P and the metallization ratio M together characterize the interdigital transducer 12 and may determine one or more operational parameters of the conventional SAW resonator 10. For example, the electrode period P and the metallization ratio M of the interdigital transducer 14, along with other factors such as the properties of the piezoelectric layer 12 may determine a series resonant frequency of the device.
In operation, an alternating electrical input signal provided at the first interdigital electrode 18A is transduced into a mechanical signal in the piezoelectric layer 12, resulting in one or more acoustic waves therein. In the case of the conventional SAW device 10, the resulting acoustic waves are predominately surface acoustic waves. As discussed above, due to the electrode period P and the metallization ratio M of the interdigital transducer 14, the characteristics of the material of the piezoelectric layer 12, and other factors, the magnitude and frequency of the acoustic waves transduced in the piezoelectric layer 12 are dependent on the frequency of the alternating electrical input signal. This frequency dependence is often described in terms of changes in the impedance and/or a phase shift between the first interdigital electrode 18A and the second interdigital electrode 18B with respect to the frequency of the alternating electrical input signal. The acoustic waves transduced by the alternating electrical input signal travel in the piezoelectric layer 12, eventually reaching the second interdigital electrode 18B where they are transduced into an alternating electrical output signal. The first reflector structure 16A and the second reflector structure 16B reflect the acoustic waves in the piezoelectric layer 12 back towards the interdigital electrode 14 to confine the acoustic waves in the area surrounding the interdigital transducer 14.
FIG. 2 is a graph illustrating an ideal relationship of the impedance (shown as admittance) and phase shift between the first interdigital electrode 18A and the second interdigital electrode 18B to the frequency of the alternating electrical input signal for the conventional SAW resonator 10. A solid line 22 illustrates the admittance between the first interdigital electrode 18A and the second interdigital electrode 18B with respect to the frequency of the alternating electrical input signal. Notably, the solid line 22 includes a peak at a first point P1 at which the admittance between the first interdigital electrode 18A and the second interdigital electrode 18B climbs rapidly to a maximum value. This peak occurs at the series resonant frequency (fS) of the conventional SAW resonator 10. The impedance between the first interdigital electrode 18A and the second interdigital electrode 18B at the series resonant frequency is minimal, such that the first interdigital electrode 18A and the second interdigital electrode 18B appear as a short-circuit. The solid line 22 also includes a valley at a second point P2 at which the admittance between the first interdigital electrode 18A and the second interdigital electrode 18B plummets rapidly to a minimum value. This valley occurs at the parallel resonant frequency (fP) of the conventional SAW resonator 10. The impedance between the first interdigital electrode 18A and the second interdigital electrode 18B at the parallel resonant frequency is at a maximum, such that the first interdigital electrode 18A and the second interdigital electrode 18B appear as an open circuit.
A dashed line 24 illustrates the phase shift between the first interdigital electrode 18A and the second interdigital electrode 18B with respect to the frequency of the alternating electrical input signal. Notably, the dashed line shows that a 90° phase shift occurs between the series resonant frequency and the parallel resonant frequency. This phase shift is due to the change in the impedance from primarily capacitive to primarily inductive between the series resonant frequency and the parallel resonant frequency.
While the series resonant frequency and the parallel resonant frequency of the conventional SAW resonator 10 are shown occurring at certain frequencies in the graph, various aspects of the conventional SAW resonator 10, such as the electrode period P and the metallization ratio M of the interdigital transducer 14, the material of the piezoelectric layer 12, and other factors may be modified to raise or lower both the series resonant frequency and the parallel resonant frequency. However, the frequency of the conventional SAW resonator 10 is generally limited due to limits in the velocity of acoustic waves in the piezoelectric layer 12. This in turn limits the utility of the conventional SAW resonator 10, precluding its use in applications requiring processing of high frequency signals above a certain threshold. Further, there are limits in the frequency delta achievable between SAW devices such as the conventional SAW resonator 10 fabricated on the same wafer, such that multi-frequency SAW devices generally must be on different wafers that consume more space in a device.
The graph shown in FIG. 2 is highly idealized. In reality, the response of the conventional SAW resonator 10 includes spurious areas that degrade the performance thereof. Further, the response of the conventional SAW resonator 10 may be temperature dependent, which may be undesirable in many circumstances. There have been numerous developments in the technology in an effort to suppress these spurious responses and temperature compensate devices, however, there is a need for further improvements in these areas.